It's well understood by good parents that life should only get so exciting for a baby.
After friends have come around and brought presents and made animated faces, after there's been some cake and some cuddles, after there've been a lot of bright lights and perhaps some songs too, enough is enough.
The baby will start to look stern, and then burst into tears, and the wise parents knows that nothing is particularly wrong, though the baby may by now be wailing.
It's just time for a nap.
The brain needs to process, digest, and divide up the wealth of experiences that have been ingested.
Boy, this hit home. It's a short video, but I kept finding myself saying "hmm, that sure makes an awful lot of sense."
I particularly liked the bit about needing to have 10 minutes to digest an hours worth of activity. I think that's why I love going on walks so much.